Sometimes you need to create a thumbnail of a PDF file for display on the web or elsewhere. Here is the ImageMagick convert command to do it.

convert \
-density 600 \ # make it high density so the resized image looks OK
example_051.pdf[2] \ # use [X] to specify the page number page 3 in this case
-resize x150 \ # resize to a height of 150 keeping the width ratio the same
-flatten \ # flatten the image so that the transparent areas aren't blacked out
out.jpg \ # the name of the output image

example_051

Refs:
tcpdf.org
http://www.binarytides.com/convert-pdf-image-imagemagick-php/